1. Field of the Invention
The present invention relates generally to methods of filtering waveform data. In another aspect, the invention concerns a method for manipulating seismic data by analyzing various phase angles of the data to thereby improve the accuracy of rock property predictions.
2. Description of the Prior Art
It is well known that certain physical properties of an object can be non-intrusively measured by transmitting various types of waves (e.g., optical, acoustical, electrical, radiation, or vibration waves) into/onto the object and detecting the waves reflected by the object. The reflected wave data can then be manipulated through various filtering and/or interpreting techniques to provide a relatively accurate indication of one or more physical properties of the object.
Two representative, but vastly different, examples of applications where the physical properties of an object are measured by reflected waves are sonograms and seismic surveys. Sonograms are typically performed on living beings by introducing acoustical waves into the body of the being. The acoustic waves are reflected differently by different body tissues or fluids. The reflected waves are detected and then manipulated in a manner which gives an indication (typically a visual image) of what types of tissues or fluids are present in particular locations of the body. Seismic surveys of the earth are performed by inducing vibrational waves (i.e., compression and/or shear waves) into the earth. The vibrational waves are reflected at different strata in the earth. The reflected waves are detected and manipulated in a manner which provides an indication of the rock properties of the subterranean formation at various locations.
A common problem encountered when using waveform data to determine properties of a physical body is that abrupt boundaries between regions of the body having different physical and acoustic properties can be “smeared” due to the nature of the waveform data. This boundary smearing of waveform data is experienced when relatively low frequency waves (e.g., sinusoidal waves) encounter an abrupt acoustic boundary between two regions having different physical properties. The abrupt boundary will typically not be represented by a spike or a single data point in the waveform data. Rather, the boundary may show up as a sinusoidal wave with one or two large peaks having smaller troughs and peaks reverberating out from the large peak(s). Thus, the boundary between physically dissimilar regions is frequently represented in waveform data by an extended wavelet which crossplots as a cluster of data points, rather than by a single data point.
It is well known to analyze seismic data using graphic or crossplot techniques, such as Amplitude Versus Offset (AVO) crossplotting. The common purpose of such crossplotting is to identify reservoir sand versus shale, or hydrocarbon-bearing sand versus wet sand. This can occur because the data representing sand and the data representing shale tend to plot in different portions of the AVO graph, with shale plotting closer towards the origin of the graph and sand tending to migrate away from the origin. A problem with such analysis of seismic data is that the graphic position of the sand is dependent upon the phase of the seismic data. With certain phases of the data, the crossplotted data representing sand located near a boundary between sand and shale is well separated from the crossplotted background shale trend. However, for certain other phases of the data, the crossplotted data representing sand located near a boundary between sand and shale may be located within the crossplotted background shale trend. Thus, certain phase angles of the seismic data provide a more accurate indication of boundary locations than other phase angles of seismic data. This problem is described in greater detail below with reference to FIGS. 6a and 6b. 